This invention relates to improvements in an ultrasonic diagnostic equipment having the dynamic focusing function.
In an ultrasonic diagnostic equipment which irradiates a subject with ultrasonic waves to take tomograms of the subject on the basis of the reflected wave signals, the dynamical changes of the focus of reception for the reflected waves, i.e., the dynamic focusing is (are) made so as to obtain at high resolution all the tomograms of various parts of the subject having unequal depths.
The focusing of the reception is made by properly delaying a plurality of received wave signals and adding the delayed signals by means of a phased array. Therefore, the dynamic focusing is realized in such a way that the individual delay magnitudes of the plurality of received signals in the phased array are altered dynamically in correspondence with the depths of generation of the reflected waves.
The alterations of the delay magnitudes in the phased array are usually made through the tap changing of delay lines which constitute the phased array. For this reason, the alteration of the focus is attended with the change-over of a switch, and a spike noise develops at the change-over of the switch. The spike noise mixes into the received reflected-wave signal and shows an image irrelevant to the subject on a tomogram display, which forms a cause for an erroneous diagnosis. One method for improving such phenomenon is to employ as the change-over switch a high-class switch which generates spike noise. Since, however, the switches for changing the delay magnitudes in the phased array are of a large number, it is uneconomical to replace all the switches with the high-class ones. On the other hand, in case where several phased arrays of unequal focal distances are prepared and are changed-over with switches, the number of the switches to be used is small, and hence, it seems economically unobjectionable to employ high-class switches. Since, however, the phased arrays themselves are far more expensive than the switches etc., it is uneconomical to dispose that number of phased arrays which is equal to the number of foci.